Role of calcium-activated potassium channels in acetylcholine-induced vasodilation of rat retinal arterioles in vivo

N-methyl-D-aspartic acid receptor-mediated vasodilation is attenuated in the retinas of diabetic rats

PURPOSE

Activation of N-methyl-d-aspartic acid (NMDA) receptors enhances nitric oxide (NO) production in retinal neuronal cells, and in turn, NO released from neuronal cells induces glial cell-mediated dilation of retinal arterioles in rats. The purpose of this study was to examine how neuronal cell-dependent, glial cell-mediated vasodilation is impacted in diabetic rat retinas.

MATERIALS AND METHODS

Diabetes was induced in 6-week-old male Wistar rats by combining streptozotocin injection and D-glucose feeding. Two weeks later, the dilator function of retinal arterioles was assessed.

RESULTS

Compared with non-diabetic rats, the dilator responses of retinal arterioles induced by intravitreal injection of NMDA and NOR3, an NO donor, were reduced in diabetic rats. Following the blockade of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels with iberiotoxin, no significant difference in the retinal vasodilator response to NOR3 was observed between non-diabetic and diabetic rats. Intravitreal injection of 14,15-epoxyeicosatrienoic acid, a vasodilatory factor released from glial cells, dilated retinal arterioles, and the response was diminished by diabetes.

CONCLUSION

These findings suggest that the impaired BK_Ca channel function in vascular cells is responsible for the diminished neuronal cell-dependent, glial cell-mediated dilation of retinal arterioles during the early stage of diabetes.

Impairment of endothelium-dependent vasodilator function of retinal blood vessels in adult rats with a history of retinopathy of prematurity

Retinopathy of prematurity (ROP) is a proliferative retinal vascular disease, initiated by delayed retinal vascular growth after premature birth. In the majority of cases, ROP resolves spontaneously; however, a history of ROP may increase the risk of long-term visual problems. In this study, we evaluated the endothelial function of retinal blood vessels in adult rats with a history of ROP. ROP was induced in rats by subcutaneous injection of a vascular endothelial growth factor receptor tyrosine kinase inhibitor (KRN633) on postnatal day (P) 7 and P8. On P56, vasodilator responses to acetylcholine, GSK1016790A (an activator of transient receptor potential vanilloid 4 channels), NOR3 (a nitric oxide [NO] donor), and salbutamol (a β2-adrenoceptor agonist) were assessed. Compared to age-matched controls, retinal vasodilator responses to acetylcholine and GSK1016790A were attenuated in P56 rats with a history of ROP. No attenuation of acetylcholine-induced retinal vasodilator response was observed under inhibition of NO synthase. Retinal vasodilator responses to NOR3 and salbutamol were unaffected. These results suggest that the production of and/or release of NO is impaired in retinal blood vessels in adult rats with a history of ROP. A history of ROP might increase the risk of impaired retinal circulation in adulthood.

L-Citrulline ameliorates the attenuation of acetylcholine-induced vasodilation of retinal arterioles in diabetic rats

In our previous study, we found that the vasodilation of retinal arterioles induced by acetylcholine and BMS-191011, a large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel opener, were diminished in diabetic rats. Currently, few agents ameliorate the impaired vasodilator responses of retinal blood vessels. Our recent finding that the intravenous infusion of L-citrulline dilated retinal arterioles, suggests that L-citrulline could be a potential therapeutic agent for circulatory disorders of the retina. In this study, we determined the effect of an oral L-citrulline treatment on impaired acetylcholine- and BMS-191011-induced vasodilation in the retinal arterioles of diabetic rats. To induce diabetes, rats were administered an intravenous dose of streptozotocin (65 mg/kg) and a 5% D-glucose solution as drinking water. The L-citrulline (2 g/kg/day) and L-arginine (2 g/kg/day) treatments commenced either 15 days before or just after the streptozotocin injection and continued throughout the experimental period. A 29-day treatment with L-citrulline, but not L-arginine, significantly ameliorated the impaired acetylcholine- and BMS-191011-induced retinal vasodilation in diabetic rats without affecting their plasma glucose levels. The 2-week L-citrulline treatment tended to ameliorate the dysfunction of the acetylcholine-induced retinal vasodilation in diabetic rats. In conclusion, these results showed that the retinal blood vessel dysfunction induced by diabetes mellitus could be prevented by the long-term administration of L-citrulline and suggest that the latter could play a potentially prophylactic role in diabetic retinopathy.

Role of Epoxyeicosatrienoic Acids in Acetylcholine-Induced Dilation of Rat Retinal Arterioles in Vivo

CYP epoxygenase-derived epoxyeicosatrienoic acids (EETs) contribute to endothelium-dependent hyperpolarization (EDH)-related dilation in multiple vascular beds. The present study aimed to determine the role of EETs in the acetylcholine (ACh)-induced dilation of retinal arterioles in rats in vivo. The vasodilator responses were assessed by determining the change in diameter of the retinal arterioles on images of the ocular fundus. The intravitreal injection of 17-octadecynoic acid (1.4 nmol/eye), an inhibitor of CYP epoxygenase, and 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EE-5(Z)-E; 2 nmol/eye), an antagonist of EETs, reduced the ACh (0.3-10 µg/kg/min)-induced dilation of the retinal arterioles. The EET antagonist attenuated the vasodilator response to ACh under blockade of nitric oxide (NO) synthases and cyclooxygenases with N^G-nitro-L-arginine methyl ester (30 mg/kg) plus indomethacin (5 mg/kg). Intravitreal injection of 14,15-EET (0.5 nmol/eye) dilated retinal arterioles and the response was prevented by iberiotoxin, an inhibitor of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels (20 pmol/eye). These results suggest that ACh stimulates the production of EETs, thereby dilating the retinal arterioles via activation of BK_Ca channels. CYP epoxygenase-derived EETs may be involved in the EDH-related component of the ACh-induced dilation of the retinal arterioles.

Involvement of Gap Junctions in Acetylcholine-Induced Endothelium-Derived Hyperpolarization-Type Dilation of Retinal Arterioles in Rats

An electrical communication between the endothelial and smooth muscle cells via gap junctions, which provides the signaling pathway known as endothelium-dependent hyperpolarization (EDH), plays a crucial role in controlling the vascular tone. In this study, we investigated the role of gap junctions in the acetylcholine (ACh)-induced EDH-type dilation of rat retinal arterioles in vivo. The dilator response was evaluated by measuring the diameter of retinal arterioles. Intravitreal injection of gap junction blockers (18β-glycyrrhetinic acid and carbenoxolone) reduced the ACh-induced dilation of retinal arterioles. Moreover, the retinal arteriolar response to ACh was attenuated by 18β-glycyrrhetinic acid under treatment with a combination of NG-nitro-L-arginine methyl ester (a nitric oxide (NO) synthase inhibitor; 30 mg/kg) and indomethacin (a cyclooxygenase inhibitor; 5 mg/kg). The NO- and prostaglandin-independent, EDH-related component of ACh-induced dilation of retinal arterioles was prevented by intravitreal injection of iberiotoxin, which inhibits large-conductance Ca2+-activated K+ channels. Furthermore, the combination of 18β-glycyrrhetinic acid and iberiotoxin produced greater attenuation in the EDH-related response than that by the individual agent. Treatment with 18β-glycyrrhetinic acid revealed no significant effect on NOR3 (an NO donor)-induced retinal vasodilator response. These results suggest that gap junctions contribute to the ACh-induced, EDH-type dilation of rat retinal arterioles in vivo.

4-Aminopyridine, a Voltage-Gated K+ Channel Inhibitor, Attenuates Nitric Oxide-Mediated Vasodilation of Retinal Arterioles in Rats

Nitric oxide (NO) is an important regulator of the retinal blood flow. The present study aimed to determine the role of voltage-gated K⁺ (K_V) channels and ATP-sensitive K⁺ (K_ATP) channels in NO-mediated vasodilation of retinal arterioles in rats. In vivo, the retinal vasodilator responses were assessed by measuring changes in the diameter of retinal arterioles from ocular fundus images. Intravitreal injection of 4-aminopyridine (a K_V channel inhibitor), but not glibenclamide (a K_ATP channel blocker), significantly attenuated the retinal vasodilator response to the NO donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Intravitreal injection of indomethacin (a non-selective cyclooxygenase inhibitor) also reduced the NOR3-induced retinal vasodilator response. The combination of 4-aminopyridine and indomethacin produced a greater reduction in the NOR3-induced response than either agent alone. 4-Aminopyridine had no significant effect on pinacidil (a K_ATP channel opener)-induced response. These results suggest that the vasodilatory effects of NO are mediated, at least in part, through the activation of 4-aminopyridine-sensitive K_V channels in the retinal arterioles of rats. NO exerts its dilatory effect on the retinal vasculature of rats through at least two mechanisms, activation of the K_V channels and enhancement of prostaglandin production.

Activation of transient receptor potential vanilloid 4 channels dilates rat retinal arterioles through nitric oxide- and BKCa channel-dependent mechanisms in vivo

Transient receptor potential vanilloid 4 (TRPV4) channel, a cation channel expressed in nearly all cell types, plays an important role in the regulation of vascular tone. In the present study, we examined the effect of GSK1016790A, an activator of TRPV4 channels, on the diameter of retinal blood vessels in rats and the underlying mechanisms. Ocular fundus images were captured with an original high-resolution digital fundus camera in vivo and diameters of retinal blood vessels were measured. Intravenous infusion of GSK1016790A (0.2-2 μg kg^-1 min^-1) increased retinal arteriolar diameter in a dose-dependent manner. The higher dose of GSK1016790A (2 μg kg^-1 min^-1) slightly decreased blood pressure. These responses to GSK1016790A were significantly attenuated by intravenous injection of GSK2193874 (0.3 mg/kg), an antagonist of TRPV4 channels. Intravitreal injection of N^ω-nitro-L-arginine methyl ester, an inhibitor of nitric oxide (NO) synthase or iberiotoxin, an inhibitor of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel, significantly attenuated the GSK1016790A-induced increases in retinal arteriolar diameter. These results suggest that activation of TRPV4 channels dilates rat retinal arterioles through NO- and BK_Ca channel-dependent mechanisms in vivo.

Differential participation of calcium-activated potassium channel in endothelium-dependent hyperpolarization-type relaxation in superior mesenteric arteries of spontaneously hypertensive rats

The purpose of this study was to determine the relationship of K_Ca channels to endothelium-dependent hyperpolarizing factor (EDHF)-mediated relaxation induced by acetylcholine (ACh) in the superior mesenteric arteries of 7-month-old spontaneously hypertensive rats (SHR). Upon inhibition of nitric oxide synthase and cyclooxygenase, ACh-induced EDHF-mediated relaxation was found to be weaker in SHR than in age-matched Wistar Kyoto rats (WKY). These relaxations in both group were attenuated by combined treatment with small-conductance and intermediate-conductance Ca²⁺-activated K⁺ channels (SK_Ca and IK_Ca) inhibitors, with the exception of relaxation resistant to inhibition of these channels in SHR (vs. WKY). Treatment with large-conductance Ca²⁺-activated K⁺ channels (BK_Ca) inhibitor specifically attenuated relaxation in SHR, but not in WKY. Protein expression of IK_Ca and SK_Ca in the arteries did not differ between the 2 groups, whereas ratio of sloβ1 subunit to α subunit of BK_Ca was increased in SHR (vs. WKY). These results suggest that EDHF-mediated relaxations in superior mesenteric arteries are impaired in SHR, and utilize components of BK_Ca in addition to SK_Ca/IK_Ca channel activities, that the increased participation of BK_Ca may be attributable to alterations in α and sloβ1 subunit ratio, and that components unrelated to K_Ca activity may also contribute to the difference between SHR and WKY arteries.

Probucol prevents the attenuation of β2-adrenoceptor-mediated vasodilation of retinal arterioles in diabetic rats

Probucol is an antihyperlipidemic drug with potent antioxidant properties. Oxidative stress plays an important role in the pathogenesis of diabetic retinopathy. In this study, we aimed to investigate the protective effects of probucol against diabetes-induced retinal vascular dysfunction in a rat model of diabetes. Diabetes was induced by a combination of streptozotocin treatment and D-glucose feeding, and retinal vasodilator responses were assessed by measuring the diameter of retinal arterioles. The vasodilator effect of salbutamol, a β₂-adrenoceptor agonist, on retinal arterioles was significantly diminished 2 weeks after the induction of diabetes. In non-diabetic rats, vasodilator responses to salbutamol were significantly reduced after an intravitreal injection of iberiotoxin, a blocker of large-conductance K_Ca (BK_Ca) channels. However, this effect was not observed in diabetic rats. Probucol had no significant effect on salbutamol-induced changes in diameter of retinal arterioles in non-diabetic rats, whereas it could prevent the attenuation of retinal vasodilator response to salbutamol in diabetic rats. These results suggest that the reduced function of BK_Ca channels is involved in the attenuation of β₂-adrenoceptor-mediated retinal vasodilation in diabetic rats. Probucol preserves the BK_Ca channel function in retinal arterioles under diabetic conditions; therefore, it may show beneficial effects on diabetic retinopathy by preventing or slowing the impairment of the retinal circulation in patients with diabetes mellitus.

Endothelial dysfunction in renal arcuate arteries of obese Zucker rats: The roles of nitric oxide, endothelium-derived hyperpolarizing factors, and calcium-activated K+ channels

The roles of nitric oxide (NO), endothelium-derived hyperpolarizing factors (EDHF), and calcium-activated K⁺ (K_Ca) channels in diabetes-associated endothelial dysfunction of small renal arteries are not clear. The present study investigated acetylcholine (ACh)-induced vasorelaxation of renal arcuate arteries from obese Zucker (OZ) rats at different diabetes durations, and the relative contribution of NO, EDHF, and K_Ca channels to the endothelial dysfunction. OZ rats of 7 weeks (prediabetic stage), 12 weeks (early diabetic stage), and 20 weeks (late diabetic stage), and time-matched lean control rats, were studied. Segments of arcuate arteries (130 to 180 μm) were isolated, cannulated and pressurized. Vascular endothelial functions were tested using ACh-induced vasodilation. Our experiments demonstrated: (1) ACh-elicited vasodilation was impaired in OZ rats of 20 weeks, but not in rats of 7 and 12 weeks; (2) inhibition of NO or EDHF (contributed by epoxyeicosatrienoic acids [EETs]) production significantly decreased ACh-induced vasodilation in both lean and OZ rats of 20 weeks. The reduction of ACh-induced vasodilation by inhibition of NO or EDHF formation was less in OZ rats, as compared to lean rats; and (3) inhibition of K_Ca channels markedly reduced ACh-induced vasodilation in lean control rats, but not in OZ rats of 20 weeks. Our observations indicated that endothelium-dependent vasodilation in renal arcuate arteries is impaired in diabetes mellitus; NO and EDHF, mainly EETs, dominate the ACh-induced vasodilation in renal arcuate arteries; the contribution of NO and EETs is impaired in diabetic rats; K_Ca channels are involved in ACh-induced vasodilation; and the activity of K_Ca channels is downregulated in diabetes mellitus.

Vasodilator Effects of Elcatonin, a Synthetic Eel Calcitonin, on Retinal Blood Vessels in Rats

The aim of this study was to examine the effects of elcatonin, a synthetic derivative of eel calcitonin, on rat retinal blood vessels, and to determine how diabetes affects the retinal vascular responses. Ocular fundus images were captured with an original high-resolution digital fundus camera in vivo. The retinal vascular responses were evaluated by measuring the diameter of retinal blood vessels contained in the digital images. Both systemic blood pressure and heart rate were continuously recorded. Elcatonin increased the diameter of retinal blood vessels but decreased mean blood pressure in a dose-dependent manner, whereas it had no significant effect on heart rate. A diminished retinal vasodilator response and significant pressor response to elcatonin were observed in rats injected intravenously with N(G)-nitro-L-arginine methyl ester, a nitric oxide (NO) synthase inhibitor. Intravitreal injection of indomethacin, a non-selective cyclooxygenase (COX) inhibitor, and SQ22536, an adenylyl cyclase inhibitor, markedly attenuated the vasodilator effects of elcatonin on retinal blood vessels. The retinal vasodilator responses to elcatonin were unaffected 2 weeks after the induction of diabetes by a combination of streptozotocin treatment and D-glucose feeding. These results suggest that elcatonin dilates rat retinal blood vessels via NO- and COX-dependent mechanisms and that the adenylyl cyclase-adenosine 3',5'-cyclic monophosphate system plays a major role in the vasodilator mechanisms. The retinal vasodilatory effects of elcatonin seem to be preserved at early stages of diabetes.

BK Channels in the Vascular System

Autoregulation of blood flow is essential for the preservation of organ function to ensure continuous supply of oxygen and essential nutrients and removal of metabolic waste. This is achieved by controlling the diameter of muscular arteries and arterioles that exhibit a myogenic response to changes in arterial blood pressure, nerve activity and tissue metabolism. Large-conductance voltage and Ca(2+)-dependent K(+) channels (BK channels), expressed exclusively in smooth muscle cells (SMCs) in the vascular wall of healthy arteries, play a critical role in regulating the myogenic response. Activation of BK channels by intracellular, local, and transient ryanodine receptor-mediated "Ca(2+) sparks," provides a hyperpolarizing influence on the SMC membrane potential thereby decreasing the activity of voltage-dependent Ca(2+) channels and limiting Ca(2+) influx to promote SMC relaxation and vasodilation. The BK channel α subunit, a large tetrameric protein with each monomer consisting of seven-transmembrane domains, a long intracellular C-terminal tail and an extracellular N-terminus, associates with the β1 and γ subunits in vascular SMCs. The BK channel is regulated by factors originating within the SMC or from the endothelium, perivascular nerves and circulating blood, that significantly alter channel gating properties, Ca(2+) sensitivity and expression of the α and/or β1 subunit. The BK channel thus serves as a central receiving dock that relays the effects of the changes in several such concomitant autocrine and paracrine factors and influences cardiovascular health. This chapter describes the primary mechanism of regulation of myogenic response by BK channels and the alterations to this mechanism wrought by different vasoactive mediators.

4-Hydroxy-2-nonenal attenuates β2-adrenoceptor-mediated vasodilation of rat retinal arterioles

4-Hydroxy-2-nonenal (4-HNE) is a major reactive aldehyde formed by lipid peroxidation, and it plays an important role in the pathogenesis of several vascular diseases, including diabetes mellitus. In this study, we examined the effects of 4-HNE on the vasodilatory mechanisms of rat retinal arterioles. The retinal vasodilator responses were assessed by measuring the diameter of retinal arterioles in the fundus images. Intravitreal injection of 4-HNE significantly prevented the vasodilation of retinal arterioles induced by the β2-adrenoceptor agonist salbutamol but not the nitric oxide donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Iberiotoxin, an inhibitor of large-conductance KCa (BKCa) channels, significantly reduced the salbutamol-induced vasodilation of retinal arterioles. The vasodilator effect of BMS-191011, a BKCa channel opener, on retinal arterioles was significantly attenuated by 4-HNE. These results suggest that 4-HNE attenuates retinal vasodilator responses to β2-adrenoceptor agonists through the impairment of the BKCa pathway. The direct effect of 4-HNE on retinal blood vessels may, therefore, contribute to the retinal vascular dysfunction observed in patients with diabetes mellitus.

Role of nitric oxide synthase isoforms for ophthalmic artery reactivity in mice

Nitric oxide synthases (NOS) are involved in regulation of ocular vascular tone and blood flow. While endothelial NOS (eNOS) has recently been shown to mediate endothelium-dependent vasodilation in mouse retinal arterioles, the contribution of individual NOS isoforms to vascular responses is unknown in the retrobulbar vasculature. Moreover, it is unknown whether the lack of a single NOS isoform affects neuron survival in the retina. Thus, the goal of the present study was to examine the hypothesis that the lack of individual nitric oxide synthase (NOS) isoforms affects the reactivity of mouse ophthalmic arteries and neuron density in the retinal ganglion cell (RGC) layer. Mice deficient in one of the three NOS isoforms (nNOS-/-, iNOS-/- and eNOS-/-) were compared to respective wild type controls. Intraocular pressure (IOP) was measured in conscious mice using rebound tonometry. To examine the role of each NOS isoform for mediating vascular responses, ophthalmic arteries were studied in vitro using video microscopy. Neuron density in the RGC layer was calculated from retinal wholemounts stained with cresyl blue. IOP was similar in all NOS-deficient genotypes and respective wild type controls. In ophthalmic arteries, phenylephrine, nitroprusside and acetylcholine evoked concentration-dependent responses that did not differ between individual NOS-deficient genotypes and their respective controls. In all genotypes except eNOS-/- mice, vasodilation to acetylcholine was markedly reduced after incubation with L-NAME, a non-isoform-selective inhibitor of NOS. In contrast, pharmacological inhibition of nNOS and iNOS had no effect on acetylcholine-induced vasodilation in any of the mouse genotypes. Neuron density in the RGC layer was similar in all NOS-deficient genotypes and respective controls. Our findings suggest that eNOS contributes to endothelium-dependent dilation of murine ophthalmic arteries. However, the chronic lack of eNOS is functionally compensated by NOS-independent vasodilator mechanisms. The lack of a single NOS isoform does not appear to affect IOP or neuron density in the RGC layer.

Protective Effect and Mechanism of Total Flavones from Rhododendron simsii Planch on Endothelium-Dependent Dilatation and Hyperpolarization in Cerebral Ischemia-Reperfusion and Correlation to Hydrogen Sulphide Release in Rats

We for the first time investigated the effect and mechanism of the total flavones of Rhododendron simsii Planch (TFR), a widely-used Chinese herb for a thousand years, on vasodilatation and hyperpolarization in middle cerebral artery (MCA) of rats subject to global cerebral ischemia-reperfusion (CIR). TFR (11~2700 mg/L) evoked dose-dependent vasodilation and hyperpolarization in MCA of both sham and CIR that were partially inhibited by 30 μM N-nitro-L-arginine-methyl-ester and 10 μM indomethacin and further attenuated by endogenous H2S synthese-CSE inhibitor PPG (100 μM) or Ca(2+)-activated potassium channel (Kca) inhibitor TEA (1 mM). In whole-cell patch clamp recording, TFR remarkably enhanced the outward current that was inhibited by TEA. CIR increased CSE mRNA expression and the contents of H2S that were further increased by TFR. We conclude that, in MCA of CIR rats, TFR induces non-NO and non-PGI2-mediated effects of vasodilatation and hyperpolarization involving Kca and increases CSE mRNA expression level in endothelial cells and H2S content in the cerebrum. These findings suggest that the response induced by TFR is potentially related to endothelium-derived hyperpolarizing factor mediated by the endogenous H2S and promote the use of TFR in protection of brain from ischemia-reperfusion injury.

Ca(2+) sparks promote myogenic tone in retinal arterioles

BACKGROUND AND PURPOSE

Ca(2+) imaging reveals subcellular Ca(2+) sparks and global Ca(2+) waves/oscillations in vascular smooth muscle. It is well established that Ca(2+) sparks can relax arteries, but we have previously reported that sparks can summate to generate Ca(2+) waves/oscillations in unpressurized retinal arterioles, leading to constriction. We have extended these studies to test the functional significance of Ca(2+) sparks in the generation of myogenic tone in pressurized arterioles.

EXPERIMENTAL APPROACH

Isolated retinal arterioles (25-40 μm external diameter) were pressurized to 70 mmHg, leading to active constriction. Ca(2+) signals were imaged from arteriolar smooth muscle in the same vessels using Fluo4 and confocal laser microscopy.

KEY RESULTS

Tone development was associated with an increased frequency of Ca(2+) sparks and oscillations. Vasomotion was observed in 40% of arterioles and was associated with synchronization of Ca(2+) oscillations, quantifiable as an increased cross-correlation coefficient. Inhibition of Ca(2+) sparks with ryanodine, tetracaine, cyclopiazonic acid or nimodipine, or following removal of extracellular Ca(2+) , resulted in arteriolar relaxation. Cyclopiazonic acid-induced dilatation was associated with decreased Ca(2+) sparks and oscillations but with a sustained rise in the mean global cytoplasmic [Ca(2+) ] ([Ca(2+) ]c ), as measured using Fura2 and microfluorimetry.

CONCLUSIONS AND IMPLICATIONS

This study provides direct evidence that Ca(2+) sparks can play an excitatory role in pressurized arterioles, promoting myogenic tone. This contrasts with the generally accepted model in which sparks promote relaxation of vascular smooth muscle. Changes in vessel tone in the presence of cyclopiazonic acid correlated more closely with changes in spark and oscillation frequency than global [Ca(2+) ]c , underlining the importance of frequency-modulated signalling in vascular smooth muscle.

Impaired acetylcholine-induced cutaneous vasodilation in young smokers: roles of nitric oxide and prostanoids

Cigarette smoking attenuates acetylcholine (ACh)-induced cutaneous vasodilation in humans, but the underlying mechanisms are unknown. We tested the hypothesis that smokers have impaired nitric oxide (NO)- and cyclooxygenase (COX)-dependent cutaneous vasodilation to ACh infusion. Twelve young smokers, who have smoked more than 5.2 ± 0.7 yr with an average daily consumption of 11.4 ± 1.2 cigarettes, and 12 nonsmokers were tested. Age, body mass index, and resting mean arterial pressure were similar between the groups. Cutaneous vascular conductance (CVC) was evaluated as laser-Doppler flux divided by mean arterial pressure, normalized to maximal CVC (local heating to 43.0°C plus sodium nitroprusside administration). We evaluated the increase in CVC from baseline to peak (CVCΔpeak) and area under the curve of CVC (CVCAUC) during a bolus infusion (1 min) of 137.5 μM ACh at four intradermal microdialysis sites: 1) Ringer (control), 2) 10 mM N(G)-nitro-l-arginine methyl ester (l-NAME; NO synthase inhibitor), 3) 10 mM ketorolac (COX inhibitor), and 4) combination of l-NAME + ketorolac. CVCΔpeak and CVCAUC at the Ringer site in nonsmokers were greater than in smokers (CVCΔpeak, 42.9 ± 5.1 vs. 22.3 ± 3.5%max, P < 0.05; and CVCAUC, 8,085 ± 1,055 vs. 3,145 ± 539%max·s, P < 0.05). In nonsmokers, CVCΔpeak and CVCAUC at the l-NAME site were lower than the Ringer site (CVCΔpeak, 29.5 ± 6.2%max, P < 0.05; and CVCAUC, 5,377 ± 1,109%max·s, P < 0.05), but in smokers, there were no differences between the Ringer and l-NAME sites (CVCΔpeak, 16.8 ± 4.3%max, P = 0.11; and CVCAUC, 2,679 ± 785%max·s, P = 0.30). CVCΔpeak and CVCAUC were reduced with ketorolac in nonsmokers (CVCΔpeak, 13.3 ± 3.6%max, P < 0.05; and CVCAUC, 1,967 ± 527%max·s, P < 0.05) and smokers (CVCΔpeak, 7.8 ± 1.8%max, P < 0.05; and CVCAUC, 1,246 ± 305%max·s, P < 0.05) and at the combination site in nonsmokers (CVCΔpeak, 15.9 ± 3.1%max, P < 0.05; and CVCAUC, 2,660 ± 512%max·s, P < 0.05) and smokers (CVCΔpeak, 11.5 ± 2.6%max, P < 0.05; and CVCAUC, 1,693 ± 409%max·s, P < 0.05), but the magnitudes were greater in nonsmokers (P < 0.05). These results suggest that impaired ACh-induced skin vasodilation in young smokers is related to diminished NO- and COX-dependent vasodilation.

Vasodilation of retinal arterioles induced by activation of BKCa channels is attenuated in diabetic rats

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels modulate the retinal vascular tone, but question of whether the impairment of the channel function contributes to abnormalities of retinal circulation has not yet been completely elucidated. The purpose of this study was to examine effects of diabetes on the vasodilation induced by activation of BK(Ca) channels. Male Wistar rats were treated with streptozotocin and experiments were performed 2 weeks later. The streptozotocin-treated animals were given drinking water containing 5% d-glucose to shorten the term in the development of retinal vascular dysfunction. The retinal vascular responses were assessed by measuring diameter of retinal arterioles in the fundus images that were captured with an original fundus camera system. In non-diabetic rats, vasodilator effects of acetylcholine on retinal arterioles were significantly reduced by iberiotoxin, an inhibitor of BK(Ca) channels. However, the inhibitory effect of iberiotoxin was not observed in diabetic rats, and the responses to the BK(Ca) channel opener BMS-191011 were almost completely abolished. The retinal vasodilator response to acetylcholine, possibly an endothelium-derived hyperpolarizing factor-mediated response, observed after treatment with N(G)-nitro-l-arginine methyl ester and indomethacin was markedly reduced in diabetic rats. The responses to pinacidil, an opener of ATP-sensitive K(+) channels, were unchanged. These results suggest that the retinal vasodilator response mediated through mechanisms involving activation of BK(Ca) channels is diminished at the early stage of diabetes in rats. The impairment of BK(Ca) channel function may contribute to abnormal retinal hemodynamics in diabetes and consequently play an important role in the pathogenesis of diabetic retinopathy.